1. Field of the Invention
The present invention relates generally to filter-press type electrodialysis apparatus in which cation exchange membranes and anion exchange membranes are alternately arranged between cell frames and spacers provided therein, and more particularly to an improved filter-press type electrodialysis apparatus which imparts excellent uniform distribution of the liquid within the diluting and concentrating cells and imparts low leakage of the liquid, from one cell through apertures within the frame to another cell, and/or discharged between the cell frames.
2. Description of the Prior Art
Filter-press type electrodialysis apparatus have been used for concentration or demineralization of electrolytes from various solutions containing electrolytes, such as for example, saline water, electroplating waste, radioactive waste, sugar solution cheese whey, organic acid solutions, or the like. In such electrodialysis apparatus, it is quite important to impart uniform distribution of the liquid to be electrodialyzed within the cell chambers and to prevent leakage of the liquid from one cell to another or from the area between the cell frames.
The uniform distribution of the liquid within the cells is closely related to the limiting current density within each of the cells (Industrial and Engineering Chemistry, Vol. 49, No. 4 page 780, 1957), and when the distribution of the liquid is different within each of the cells, the corresponding limiting current density is presented to each cell. Consequently, the limiting current density of the apparatus is controlled at the lowest value of the various limiting current densities and accordingly, the efficiency of such an apparatus is low. It has therefore been attempted to impart uniform distribution of the liquid within the cells, however, it has been relatively difficult to maintain such uniform distribution of the liquid within a commercial scale apparatus.
Similarly, leakage of the liquid between cells causes contamination of the dialyzed liquid whereby the electrodialysis operation itself becomes meaningless. Accordingly, it is quite important to prevent leakage of the liquid, and various proposals dealing with the liquid feeding or discharging means of the cells, which are the passages connecting the central opening portions of the cell frames to conduit apertures within the cell frames, have been made.
For example, the following structures have been known:
1. A portion of the cell frame, leading from the conduit apertures of the cell frame to the central opening, is deleted so as to form a passage for the liquid within a narrow area.
2. A portion of the cell frame, leading from the conduit apertures of the cell frame to the central opening, is deleted and spacers made of rough fabric and having a thickness not less than the thickness of the cell frame, are disposed within the deleted portion of the cell frame.
3. A portion of the cell frame leading from the conduit apertures of the cell frame to the central opening is deleted as in the instance of the apparatus of (2) above and an elastic material having projections which have a thickness not less than the thickness of the cell frame is disposed within the deleted portion.
However, these conventional contrivances could nevertheless not impart uniform distribution of the liquid to the cells or could not treat a sufficient amount of the liquid within an operative cycle of the apparatus with high efficiency. When such proposals have been applied to a commercial scale apparatus, the purposes and goals could not be accomplished to an extent which would sufficiently obviate the problem. Especially within the instance of the apparatus of (3) above, the passage for the liquid is formed by a contraction of the elastic projections. Accordingly, the passages of the liquid are different within each of the cells, and obviously are not uniform. Moreover, in commericial scale apparatus, it is especially difficult to uniformly secure all of the cell chambers within the apparatus, and under such conditions, the distribution of the liquid within the cell chambers is quite non-uniform, and the limiting current density which indicates the level of safe operation of the apparatus, fluctuates among the cells and it is quite disadvantageous for efficient operation of the electrodialysis apparatus.